Magnetic memory structures using electric-field controlled interlayer exchange coupling (IEC) for magnetization switching

ABSTRACT

A magnetic memory structure employs electric-field controlled interlayer exchange coupling between a free magnetic layer and a fixed magnetic layer to switch a magnetization direction. The magnetic layers are separated by a spacer layer disposed between two oxide layers. The spacer layer exhibits a large IEC while the oxide layers provide tunnel barriers, forming a quantum-well between the magnetic layers with discrete energy states above the equilibrium Fermi level. When an electric field is applied across the structure, the tunnel barriers become transparent at discrete energy states via a resonant tunneling phenomenon. The wave functions of the two magnets then can interact and interfere to provide a sizable IEC. IEC can control the magnetization direction of the free magnetic layer relative to the magnetization direction of the fixed magnetic layer depending on the sign of the IEC, induced by a magnitude of the applied electric field above a threshold value.

GOVERNMENT SUPPORT

This invention was made with government funds under Agreement No.HR0011-18-3-0004 awarded by The Defense Advanced Research ProjectsAgency (DARPA). The U.S. Government has certain rights in thisinvention.

FIELD OF THE DISCLOSURE

The technology of the disclosure relates generally to electronic datastorage and, more particularly, to data storage in a magnetic memorystructure.

BACKGROUND

Many different technologies are used to create electronic data storage(“memory”) elements for storing programs and data in binary form inelectronic devices. Memory element technologies are characterized bytheir access times (i.e., read and write delay), density, powerutilization, and volatility in the absence of power. Technologies thatexcel in one or more of these characteristics are typically inferior inother respects. As a result, different memory element technologies areused for different purposes, according to their particularcharacteristics. FIG. 1 is a graph 100 which compares read/write speedand density of various electronic data storage technologies. As shown inFIG. 1, static random access memory (RAM) (SRAM) technology can have avery fast read/write speed, which is beneficial for use with ahigh-speed processor, but the high speed is obtained at the expense of alarge area needed for the multiple transistors of which each SRAM cellis comprised, as well as the other components of an SRAM array. DynamicRAM (DRAM) is more dense, but not as fast as high-speed SRAM.Unfortunately, both SRAM and DRAM elements are volatile, meaning thatthe data stored therein is lost when power is removed. In contrast, ahigh-density non-volatile memory (NVM) such as NAND FLASH has beendeveloped for long-term storage, but the long access time of NVM storageis too slow for use in high-speed processor applications. Field-writtenmagnetic RAM (MRAM) is an example of memory that can have a much fasterread/write speed (i.e., short access latency) than NAND FLASH, but isnon-volatile, so no power is required to preserve the data storedtherein.

Data is stored in an MRAM by controlling a direction of magnetization ina free magnetic layer in a magnetic structure. A binary bit is stored asone of two magnetization directions. FIG. 2 is a graph 200 comparingenergy consumption and access latency of existing MRAM technologiesbased on spin-transfer torque (STT), spin-orbit torque (SOT), and STTwith voltage controlled magnetism (VCM) to those of SRAM. Although MRAMis non-volatile, the access latencies and energy consumption levels ofthe MRAM technologies are orders of magnitude higher than those of SRAM.Existing MRAM technologies require high current density for switching amagnetization direction of the free magnetic layer, causing high powerconsumption and less bit density than is preferred for mobile devices.

SUMMARY

Aspects disclosed herein include electric-field controlled interlayerexchange coupling (IEC) for magnetization switching. Electronic datastored in a magnetic memory structure can remain stable for a longperiod of time in the absence of a power source, unlike static randomaccess memory (RAM) (SRAM) and dynamic RAM (DRAM) which lose data whenpower is removed. Existing magnetic RAM (MRAM) technologies employingcurrent-induced spin-transfer torque (STT) or spin-orbit toque (SOT),however, require a large current density for switching a magnetizationdirection, which limits advancements in energy efficiency and bitdensity. A magnetic memory structure disclosed herein makes it possibleto employ IEC between a free magnetic layer and a fixed magnetic layerto switch a direction of magnetization in the free magnetic layer underthe control of an applied electric field with a significantly lowercurrent density than is required in STT or SOT MRAM devices. Themagnetic layers are separated by a composite layer including a spacerlayer disposed between two oxide layers. The spacer layer exhibits alarge IEC while the oxide layers provide tunnel barriers. Thecombination of a spacer layer between oxide layers forms a quantum-well(QW) between the magnetic layers with discrete energy states above theequilibrium Fermi level. When an electric field is applied in adirection across the magnetic memory structure, the tunnel barriersbecome transparent to the wave functions of the magnetic layers at thelevels of the discrete energy states via a resonant tunnelingphenomenon. This transparency allows the IEC between the fixed magneticlayer and the free magnetic layer to control the magnetization directionof the free magnetic layer to be parallel or antiparallel to themagnetization direction of the fixed magnetic layer depending on themagnitude of the applied electric field in the direction of the electricfield. Different magnitudes of an electric field induce different signs(i.e., polarities) of IEC. Here, a positive sign of IEC refers to aswitching that will yield an antiparallel alignment, and a negative signof IEC refers to a switching that will yield a parallel alignment. TheIEC can be measured as an energy density in units of Joules per squaremeter. A magnitude of the IEC required to achieve magnetizationswitching is defined by a threshold value. The IEC threshold value isindependent of a parameter describing a Gilbert damping and a parameterfor indicating magnets with in-plane or perpendicular anisotropies,which is not true of existing MRAM technologies. The IEC threshold valuecan be controlled by selection of materials, cross-sectional area, andthicknesses of the composite layer. However, the switching time of themagnetic memory structure herein can be optimized by the Gilbert dampingand depends on whether the magnet has an in-plane or a perpendicularanisotropy, similar to the existing spin-torque based mechanisms.

In this regard, in one aspect, a magnetic memory structure is disclosed.The magnetic memory structure includes a fixed magnetic layer, a freemagnetic layer, and a composite layer disposed between the fixedmagnetic layer and the free magnetic layer. A magnetization direction ofthe free magnetic layer relative to a magnetization direction of thefixed magnetic layer is parallel in response to a first input voltage ofa polarity between the fixed magnetic layer and the free magnetic layer.The magnetization direction of the free magnetic layer relative to themagnetization direction of the fixed magnetic layer is antiparallel inresponse to a second input voltage of the polarity between the fixedmagnetic layer and the free magnetic layer.

In another aspect, a magnetic memory structure is disclosed. Themagnetic memory structure includes a fixed magnetic layer, a freemagnetic layer, and a composite layer disposed between the fixedmagnetic layer and the free magnetic layer. In the magnetic memorystructure, a magnetization direction of the free magnetic layer relativeto a magnetization direction of the fixed magnetic layer corresponds toan IEC in the composite layer based on a magnitude of an input voltagebetween the fixed magnetic layer and the free magnetic layer.

In another aspect, a magnetic memory structure including a fixedmagnetic layer, a free magnetic layer, and a composite layer disposedbetween the fixed magnetic layer and the free magnetic layer isdisclosed. The composite layer includes a spacer layer disposed betweenthe fixed magnetic layer and the free magnetic layer, a first oxidelayer disposed between the fixed magnetic layer and the spacer layer,and a second oxide layer disposed between the spacer layer and the freemagnetic layer. In the magnetic memory structure, a first magnetizationdirection of the free magnetic layer relative to a magnetizationdirection of the fixed magnetic layer corresponds to a first magnitudeof an input voltage of a polarity between the fixed magnetic layer andthe free magnetic layer. A second magnetization direction of the freemagnetic layer relative to the magnetization direction of the fixedmagnetic layer corresponds to a second magnitude of the input voltage ofthe polarity between the fixed magnetic layer and the free magneticlayer. The first magnitude of the input voltage and the second magnitudeof the input voltage depend on a thickness of the spacer layer.

In another aspect, a method of storing data in a magnetic memorystructure is disclosed. The method includes supplying a first inputvoltage of a polarity between a fixed magnetic layer and a free magneticlayer separated by a composite layer to control a magnetizationdirection of the free magnetic layer to be parallel to a magnetizationdirection of the fixed magnetic layer. The method also includessupplying a second input voltage of the polarity between the fixedmagnetic layer and the free magnetic layer to control the magnetizationdirection of the free magnetic layer to be antiparallel to themagnetization direction of the fixed magnetic layer.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure and,together with the description, serve to explain the principles of thedisclosure.

FIG. 1 is a graph comparing read/write speed and density of variouselectronic data storage technologies;

FIG. 2 is a graph comparing energy consumption and access latency ofexisting magnetic random access memory (RAM) (MRAM) technologies tothose of static RAM (SRAM);

FIG. 3 is a schematic diagram of a magnetic memory structure thatemploys current induced spin-transfer torque (STT) to set amagnetization direction of a free magnetic layer relative to amagnetization direction of a fixed magnetic layer that is separated fromthe free magnetic layer by an oxide layer;

FIG. 4 is a schematic diagram of a magnetic memory structure thatemploys current induced spin-orbit torque (SOT) from a spin-orbitmaterial to set a magnetization direction of a free magnetic layerrelative to a magnetization direction of a fixed magnetic layer that isseparated from the free magnetic layer by an oxide layer;

FIG. 5A is a schematic diagram of a magnetic structure includingmagnetic layers separated by an oxide layer, and which may employ theSTT method of FIG. 3, and includes an illustration of the energy barrierabove the Fermi level created by the oxide layer;

FIG. 5B is a graph illustrating that interlayer exchange coupling (IEC)across the oxide layer decreases quickly to a negligible level asthickness of the oxide layer increases;

FIG. 6A is a schematic diagram of a magnetic structure includingmagnetic layers separated by a spacer layer, and includes anillustration of the spin-dependent quantum-well (QW) formed in thespacer layer with discrete states below the equilibrium Fermi level;

FIG. 6B is a graph illustrating that polarity of the IEC between thefixed magnetic layer and the free magnetic layer oscillates as afunction of the thickness of the spacer layer, and oscillatory peaks ofthe IEC decrease in magnitude with increased spacer thickness. For aparticular spacer layer thickness, the free magnetic layer is eitherparallel or antiparallel to the fixed magnetic layer, if the IEC sign isnegative or positive, respectively;

FIG. 7A is a schematic diagram of a magnetic memory structure includinga fixed magnetic layer and a free magnetic layer separated by acomposite layer including a spacer layer sandwiched between two oxidelayers, and includes an illustration of the barriers created by theoxide layers and the discrete energy states in the QW, above the Fermilevel, created by the magnetic memory structure. The discrete energystates below the Fermi level in the spacer layer is due to thespin-dependent QW formed because of the mismatch of majority and/orminority spin bands at the magnetic interfaces;

FIG. 7B is a graphic illustration of the IEC in the composite layer inFIG. 7A oscillating between positive and negative values and the IECpeaks increasing in magnitude with an increase in an electric fieldapplied in a direction;

FIG. 8A is the graph in FIG. 7B illustrating that the first peak in theoscillating IEC is below a threshold level for switching a magnetizationdirection of the free layer;

FIG. 8B includes a graph of a simulation of voltage pulses in adirection across the magnetic memory structure where the voltages ineach pulse correspond to the peak voltages shown in FIG. 8A, andincludes a corresponding graph of the magnetization directions of thefixed magnetic layer and the free magnetic layer over time;

FIG. 9 shows equations for determining (i) switching threshold, and (ii)switching speed in an STT device;

FIG. 10 shows equations for determining (i) switching threshold, and(ii) switching speed in the magnetic memory structure of FIG. 7A;

FIG. 11A is a graphical representation showing that peaks of the inputvoltage corresponding to oscillatory peaks of the IEC are observed at alower input voltage as the width of the QW increases;

FIG. 11B is a graphical representation showing that the input voltagerequired to maintain a particular magnitude of IEC increases with theheight of the QW;

FIGS. 12-14 are schematic diagrams of examples of respective circuitsthat can be used for determining a state of (e.g., reading) the magneticmemory structure in FIG. 7A.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

Aspects disclosed herein include electric-field controlled interlayerexchange coupling (IEC) for magnetization switching. Electronic datastored in a magnetic memory structure can remain stable for a longperiod of time in the absence of a power source, unlike static randomaccess memory (RAM) (SRAM) and dynamic RAM (DRAM) which lose data whenpower is removed. Existing magnetic RAM (MRAM) technologies employingcurrent-induced spin-transfer torque (STT) or spin-orbit toque (SOT),however, require a large current density for switching a magnetizationdirection, which limits advancements in energy efficiency and bitdensity. A magnetic memory structure disclosed herein makes it possibleto employ IEC between a free magnetic layer and a fixed magnetic layerto switch a direction of magnetization in the free magnetic layer underthe control of an applied electric field with a significantly lowercurrent density than is required in STT or SOT MRAM devices. Themagnetic layers are separated by a composite layer including a spacerlayer disposed between two oxide layers. The spacer layer exhibits alarge IEC while the oxide layers provide tunnel barriers. Thecombination of a spacer layer between oxide layers forms a quantum-well(QW) between the magnetic layers with discrete energy states above theequilibrium Fermi level. When an electric field is applied in adirection across the magnetic memory structure, the tunnel barriersbecome transparent to the wave functions of the magnetic layers at thelevels of the discrete energy states via a resonant tunnelingphenomenon. This transparency allows the IEC between the fixed magneticlayer and the free magnetic layer to control the magnetization directionof the free magnetic layer to be parallel or antiparallel to themagnetization direction of the fixed magnetic layer depending on themagnitude of the applied electric field in the direction of the electricfield. Different magnitudes of electric field induce different signs(i.e., polarities) of IEC. Here, a positive sign of IEC refers to aswitching that will yield an antiparallel alignment, and a negative signof IEC refers to a switching that will yield a parallel alignment. TheIEC can be measured as an energy density in units of Joules per squaremeter. A magnitude of the IEC required to achieve magnetizationswitching is defined by a threshold value. The IEC threshold value isindependent of a parameter describing a Gilbert damping and a parameterfor indicating magnets having either in-plane or perpendicularanisotropies of the free magnetic layer, unlike existing MRAMtechnologies. The threshold value can be controlled by selection ofmaterials, cross-sectional area, and thicknesses of the composite layer.However, the switching time of the magnetic memory structure herein canbe optimized by the Gilbert damping and magnetic anisotropy.

To provide a context for the discussion of the magnetic memory structureillustrated in FIG. 7A, existing magnetic memory structures and themethods used for switching a magnetization direction in those structuresare first discussed with reference to FIGS. 3-6B. FIG. 3 is a schematicdiagram of a magnetic memory structure 300 that employs STT to control amagnetization direction of a free magnetic layer 304 relative to amagnetization direction of a fixed magnetic layer 302 that is separatedfrom the free magnetic layer 304 by an oxide layer 306. Themagnetization direction of the free magnetic layer 304 can be setparallel to the magnetization direction of the fixed magnetic layer 302by a current I_(STT) through the oxide layer 306 in a first direction.The magnetization direction of the free magnetic layer 304 can be setantiparallel to a direction of the fixed magnetic layer by reversing thedirection of the current I_(STT). Due to the high current densityrequired to change a magnetization direction using STT, a large amountof power is consumed. The large power requirement limits packagingdensity, and the high current density can cause reliability problems bybreaking down the oxide layer 306. A problem with STT MRAM technology isthat the write time for switching the magnetization direction isinversely related to the write current required to switch the device. Inother words, to decrease the switching time (e.g., reduce write accesslatency), the write current must be increased. Thus, reducing the largewrite current to save power and/or increase density results in slowerwrite access times.

FIG. 4 is a schematic diagram of a magnetic memory structure 400 thatemploys SOT to control a magnetization direction of a free magneticlayer 404 relative to a magnetization direction of a fixed magneticlayer 402 separated from the free magnetic layer 404 by an oxide layer406. To set the magnetization direction of the free magnetic layer 404(i.e., to write data into the magnetic memory structure 400), a currentI_(SOT) is passed through a spin-orbit material 408 on a planar side ofthe free magnetic layer 404 opposite to the oxide layer 406. The currentrequired to write to a memory element using SOT is less than the currentrequired for STT, but the associated power consumption is stillprohibitive and limits bit density. In addition, an SOT memory structure400 is a three-terminal device, which further limits increases in bitdensity.

FIG. 5A is a schematic diagram of a magnetic memory structure 500including magnetic layers 502 and 504 separated by an oxide layer 506wherein the STT or SOT methods of FIGS. 3 and 4 may be employed, and anillustration 508 of an energy barrier 510 created by the oxide layer 506above the Fermi level E_(F). The energy barrier 510 illustrated in FIG.5A has a height Δ_(BO) that represents an energy barrier that must beovercome for switching magnetization by the STT or SOT methods. FIG. 5Bis a graph 512 illustrating IEC, which is a coupling of the wavefunctions of the respective magnetic layers across the oxide layer 506.In the graph 512, the IEC decreases to a negligible level in the oxidelayer 506 as the oxide thickness D_(O) increases from a few tenths(e.g., 0.3) of a nanometer (nm) to 1 nm and higher. At a smallerthickness, a non-negligible IEC is possible via a tunneling phenomenon,but IEC due to the wave functions of the fixed magnetic layer 502 andthe free magnetic layer 504 does not exist in the oxide layer 506 havinga thickness above 1 nm.

FIG. 6A is a schematic diagram of a magnetic memory structure 600including a fixed magnetic layer 602 and a free magnetic layer 604separated by a non-magnetic spacer layer 606. FIG. 6A also includes anillustration 608 of the discrete energy levels of electrons below theequilibrium Fermi level E_(F) in the spacer layer 606. Wave functions ofthe magnetic layers 602 and 604 exist within the spacer layer 606 due tothe type of non-magnetic material of which it is formed. Quantuminterference between the wave functions of the respective magneticlayers 602 and 604 can be constructive or destructive. The illustration608 is a representation of majority/minority spin-dependentquantum-wells (QWs) with discrete energy levels within the spacer layer606 at equilibrium.

As shown in FIG. 6B, the polarity of the IEC (having magnitude J_(EX))between the fixed magnetic layer 602 and the free magnetic layer 604oscillates as a function of the spacer thickness D_(S) of the spacerlayer 606, and the oscillatory IEC peaks decrease in magnitude withincreased spacer thickness. Due to the IEC, the magnetization directionof the free magnetic layer 604 will adopt a ferromagnetic orantiferromagnetic state with respect to the magnetization direction ofthe fixed magnetic layer 602, depending on the thickness of the spacerlayer 606.

FIG. 7A is a schematic diagram of a magnetic memory structure 700including a fixed magnetic layer 702 and a free magnetic layer 704separated by a composite layer 706. The composite layer 706 includes aspacer layer 708 and two oxide layers 710. The spacer layer 708 isdisposed between the fixed magnetic layer 702 and the free magneticlayer 704. A first oxide layer 710 is disposed between the fixedmagnetic layer 702 and the spacer layer 708, and a second oxide layer710 is disposed between the spacer layer 708 and the free magnetic layer704. FIG. 7A includes an illustration 711 of barriers 712 created by theoxide layers 710. The illustration 711 also shows the discrete energystates 714 that are made possible in a QW 716 formed in the compositelayer 706 in the presence of an applied electric field. The discreteenergy states 714 are above the Fermi level E_(F). The QW 716 is formedin the composite layer 706 between the fixed and free magnetic layers702 and 704. Below the equilibrium Fermi level, majority/minorityspin-dependent QWs with discrete energy levels exist within the spacerlayer 708.

The oxide layers 710 create the barriers 712 having a height Δ_(BC)above the Fermi level E_(F). The barriers 712 impede the flow ofelectrons across the spacer layer 708, forming the QW 716. However, dueto resonant tunneling phenomena, a transmission coefficient across theoxide layers 710 is increased at the discrete energy states 714 to allowa stronger interaction between the wave functions of the fixed and freemagnetic layers 702 and 704. The energy states 714 in the QW 716 may beprobed by contact electrochemical potential with an electric field froman input voltage V_(in) applied across the magnetic memory structure700. A threshold voltage V_(th) is a voltage required to achieve hightransmission across the oxide layers 710 via the discrete energy states714 within the QW 716, to induce an IEC strength greater than theswitching threshold value. A magnitude or strength of the IEC depends onthe wave function interaction at all quantum energy states in the QW716, including those below the Fermi level E_(F), and those above theFermi level E_(F) achieved by the application of the electric fieldbased on the input voltage V. The current density in the structuredepends only on the quantum energy states in the QW 716, within theseparation between two contact Fermi levels. Consequently, the currentdensity flowing in the structure is much lower than in the STT or SOTmethods, when a voltage or an electric field is applied to achieve anIEC strength greater than the threshold value.

The magnetic memory structure 700 also includes a first electrode 718electrically coupled to the fixed magnetic layer 702 and a secondelectrode 720 electrically coupled to the free magnetic layer 704. Theinput voltage V_(in) is applied across the first and second electrodes718 and 720. The magnetic memory structure 700 may be included in amagnetic memory device (not shown).

FIG. 7B is a graphic illustration of the IEC (having a magnitude J_(EX))in the composite layer 706 in FIG. 7A oscillating between positive andnegative values and shows the IEC peaks increasing in magnitude with anincrease in the electric field applied in a direction based on the inputvoltage V. The IEC increases significantly when the input voltage V_(in)causes the electron energy within the QW 716 to achieve one of theenergy states 714. The sign of the IEC changes polarity between peaks(i.e., at each energy state 714 reached due to the applied electricfield). Therefore, the IEC oscillates between positive and negativevalues that increase in magnitude with an applied input voltage V. Inthe example in FIG. 7B, a voltage of 1.0 volts (V) achieves a first IECenergy peak of positive polarity. As discussed below, in this example,the energy states 714 of the QW 716 also help reach a second and a thirdIEC peak, which are negative and positive peaks, respectively, at aninput voltage V_(in) of 1.25 V and 1.5 V.

FIG. 8A is the graphic illustration in FIG. 7B with labels at voltagelevels P₁-P₄. The input voltage V_(in) applied to the magnetic memorystructure 700 in FIG. 7A at voltage level P₁ in FIG. 8A is 0 V. Sincethere is negligible transmission of electron wave functions across theoxide barriers, IEC at voltage level P₁ is also negligible. In theexample in FIG. 8A, when the voltage V_(in) is raised to 1.0 V at P₂, asizable positive IEC is achieved via energy states of the QW 716. With apositive IEC at the voltage level P₂, there should be ananti-ferromagnetic coupling between the free magnetic layer 704 and thefixed magnetic layer 702. However, magnetization switching does notoccur in the magnetic memory structure 700 until the magnitude of theIEC reaches a switching threshold, which is defined by a threshold valueof an energy density of IEC. The switching threshold is attained by theinput voltage V_(in) shown in the illustration 711 in FIG. 7A. In otherwords, the magnitude of the IEC is below the switching threshold at thevoltage of 1.0 volts at voltage level P₂, so the IEC is not strongenough to cause the direction of magnetization in the free magneticlayer 704 to be switched by the magnetization of the fixed magneticlayer 702. As the voltage V_(in) is increased from 1.0 V, the IECmagnitude initially decreases and then becomes an increasing negativevalue. When the input voltage V_(in) reaches 1.25 V at voltage level P₃,another IEC peak with negative polarity is achieved via energy states714 of the QW 716. With a negative IEC, there should be a ferromagneticcoupling between the free magnetic layer 704 and the fixed magneticlayer 702. The magnitude of the IEC is above the switching threshold atvoltage level P₃, and the IEC has a negative value, so ferromagneticcoupling causes the direction of magnetization of the free magneticlayer 704 to switch to become parallel to the direction of magnetizationof the fixed magnetic layer 702, or causes the direction ofmagnetization of the free magnetic layer 704 to remain parallel to thedirection of magnetization of the fixed magnetic layer 702 if alreadyparallel. When the input voltage V_(in) reaches 1.5 V at voltage levelP₄, another IEC peak with positive polarity is achieved via energystates 714 of the QW 716. With a positive IEC, there should be ananti-ferromagnetic coupling between the free magnetic layer 704 and thefixed magnetic layer 702. At voltage level P₄, with a positive IEC thatis above the switching threshold, the direction of magnetization of thefree magnetic layer 704 will switch to become antiparallel to thedirection of the magnetization of the fixed magnetic layer 702 or thedirection of magnetization of the free magnetic layer 704 will remainantiparallel to the direction of magnetization of the fixed magneticlayer 702 if already antiparallel.

Unlike magnetization switching by STT or SOT, employingvoltage-controlled IEC to switch a magnetization direction of the freemagnetic layer 704 in the magnetic memory structure 700 does not requirea change in the polarity of the applied voltage. The switching does notdepend on the polarity of the applied voltage but depends on themagnitude. This aspect is shown in the results of a simulationillustrated in FIG. 8B. FIG. 8B includes a graph of a simulation ofpulses of the input voltage V_(in) of a polarity (e.g., positive ornegative between the fixed magnetic layer 702 and the free magneticlayer 704) creating an electric field in a direction (e.g., through thecomposite layer 706) across the magnetic memory structure 700. Thevoltage levels of the pulses in FIG. 8B correspond approximately withthe voltage levels P₄, P₃, and P₂ shown in FIG. 8A. FIG. 8B includes acorresponding graph of the magnetization directions of the fixedmagnetic layer 702 and the free magnetic layer 704 over time t.

In the example in FIG. 8B, the simulation starts at time t=0 nanoseconds(ns) with the input voltage V_(in)=0 V, at which there is negligible IECin the composite layer 706, and an initial condition in which thedirections of magnetization of the respective magnetic layers 702 and704 are parallel to each other (e.g., both at magnetization of 1). TheIEC is negligible at time 0 due to the barriers 712 created by the oxidelayers 710 in FIG. 7A. At an input voltage V_(in) of 0 V, the directionof magnetization of the free magnetic layer 704 will remain stable.

At time t=10 ns, in the example in FIG. 8B, the input voltage V_(in) israised to 1.6 V, which corresponds to a positive IEC(anti-ferromagnetic) and the magnitude is above the switching threshold.After a minimum switching time T_(IEC) (not shown), the IEC causes thedirection of the magnetization of the free magnetic layer 704 to switchfrom being parallel to being antiparallel to the direction ofmagnetization of the fixed magnetic layer 702. After a certain timegreater than the minimum switching time T_(IEC), the input voltageV_(in) returns to 0 V, at which the direction of magnetization of thefree magnetic layer 704 will remain stable until application of anotherinput voltage V_(in) greater than the threshold voltage V_(th).

At time t=25 ns, in the example in FIG. 8B, the input voltage V_(in) israised to 1.3 V, which corresponds to a negative IEC (ferromagnetic) andthe magnitude is above the switching threshold. After the minimumswitching time T_(IEC), the IEC causes the direction of themagnetization of the free magnetic layer 704 to switch back from beingantiparallel to being parallel to the direction of the magnetization ofthe fixed magnetic layer 702. After a certain time greater than theminimum switching time T_(IEC), the input voltage V_(in) returns to 0 V,where the direction of magnetization of the free magnetic layer 704 willremain stable until application of another input voltage V_(in) greaterthan the threshold voltage V_(th).

At time t=40 ns, in the example in FIG. 8B, the input voltage V_(in) israised from 0 V to 1.0 V, which corresponds to a positive IEC(anti-ferromagnetic), but the magnitude of the IEC is below theswitching threshold. Thus, the direction of magnetization of the freemagnetic layer 704 remains unchanged (i.e., parallel to the direction ofmagnetization of the fixed magnetic layer 702).

The voltage levels P₃ and P₄ are shown in FIG. 8B as square waves havinginstantaneous rise times and fall times. In practice, the rise times ofthe input voltage V_(in) should be approximately 10 picoseconds (ps) andthe fall times should be approximately 10 ps. The input voltage V_(in)corresponding to voltage level P₄ at a higher energy state 714 of the QW716 should be reduced quickly to 0 V to avoid unintentionally changingthe magnetization direction. For example, if the input voltage V_(in) israised to 1.6 V to set the magnetization directions to be antiparallel,and the voltage then decreases slowly, such that the input voltageV_(in) is near 1.3 V for at least the minimum switching time T_(IEC),the magnetization of the free layer 704 could unintentionally return toa parallel state (e.g., corresponding to voltage level P₃).

As shown in FIG. 8B, in the magnetic memory structure 700 in FIG. 7A,the magnetization direction of the free magnetic layer 704 relative tothe magnetization direction of the fixed magnetic layer 702 becomesantiparallel in response to a first input voltage V_(in) (e.g., 1.6 V atpoint P₄) having a polarity between the fixed magnetic layer 702 and thefree magnetic layer 704, and parallel in response to a second inputvoltage V_(in) (e.g., 1.3 V at point P₃) having the same polaritybetween the fixed magnetic layer 702 and the free magnetic layer 704,wherein a magnitude of the first input voltage V_(in) is different thana magnitude of the second input voltage V_(in). In other words, thepolarity of the first input voltage V_(in) is the same as the polarityof the second input voltage V_(in) and both may be either positive ornegative.

The first input voltage V_(in) and the second input voltage V_(in) areeach applied between the first electrode 718 and the second electrode720 of the magnetic memory structure 700 in FIG. 7A. The first inputvoltage V_(in) and the second input voltage V_(in) increase the IECbetween the fixed magnetic layer 702 and the free magnetic layer 704above a threshold at which the magnetization direction of the freemagnetic layer 704 may be changed by the magnetization direction of thefixed magnetic layer 702.

In one example, the magnetic memory structure 700 is configured toswitch the magnetization direction of the free magnetic layer 704relative to the magnetization direction of the fixed magnetic layer 702from antiparallel to parallel in response to the first input voltageV_(in), and from parallel to antiparallel in response to the secondinput voltage V.

As discussed above, the magnetization direction of the free magneticlayer 704 may be switched by IEC in the presence of a voltage-controlledelectric field corresponding to a discrete energy state 714 of the QW716 formed by the composite layer 706. The pulses of the input voltageV_(in) illustrated in FIG. 8B provide an IEC with a magnitude above aswitching threshold and a duration of at least a minimum switching timeT_(IEC) required for switching the magnetization direction. The IECstrength (magnitude) required for magnetization switching is defined bya current density. As noted above with regard to the examples in FIGS. 3and 4, in which the magnetization of the magnetic memory structures 300and 400 are switched by STT and SOT, respectively, it was noted thatthose methods require a large current density. The large current densitycauses excessive power consumption, breaks down the oxide layer, andprevents a high bit density. The write time for switching themagnetization direction using STT is inversely related to the writecurrent required to switch the device. This relationship is shown in theequations in FIG. 9.

FIG. 9 shows (i) a switching threshold equation for determining acurrent density J_(STT0) and (ii) a switching speed equation fordetermining a minimum switching time T_(s0). The current densityJ_(STT0) and switching time T_(s0) are required for switching amagnetization direction using STT. Each of the switching thresholdequation and the switching speed equation includes the Gilbert dampingfactor α_(g), which creates a trade-off due to an inverse relationshipbetween the current density J_(STT0) and switching time T_(s0). Forexample, to reduce the large current density J_(STT0), to avoid highpower consumption, etc., the Gilbert damping factor α_(g) could bereduced, but a reduction in the Gilbert damping factor α_(g) wouldincrease the switching time T_(s0).

In contrast to the equations in FIG. 9 for magnetization switching usingSTT, the corresponding equations for magnetization switching employingvoltage controlled IEC are shown in FIG. 10. It is significant to notethat, while the switching speed equation in FIG. 10 includes the Gilbertdamping factor α_(g), the Gilbert damping factor α_(g) is not includedin the switching threshold equation in FIG. 10. A voltage requiredbetween the fixed magnetic layer 702 and the free magnetic layer 704(i.e., an input voltage V_(in) applied between the first and secondelectrodes 718 and 720) is determined by a total current I_(SW), whichcan be determined by the product of the surface area S of the magneticmemory structure 700 and the IEC energy density J_(ex), which providesthe threshold IEC required for magnetization switching therein. The IECenergy threshold J_(ex0) satisfies the equation:J _(IEC0)=(E ₁ ×E ₂)/(S(E ₁ +E ₂)), wherein:

J_(IEC0)=a minimum energy density threshold of the IEC needed forswitching the magnetization direction of the free magnetic layer, and ismeasured in units of Joule per square meter;

S=the cross-sectional area of the magnetic memory structure;

E₁=the thermal energy barrier of the fixed magnetic layer, whichsatisfies the equation:E ₁=(M _(s1) H _(k1)Ω₁)/2, wherein:

M_(s1)=saturation magnetization of the fixed magnetic layer;

H_(k1)=anisotropy field of the fixed magnetic layer;

Ω₁=volume of the fixed magnetic layer; and

E₂=the thermal energy barrier of the free magnetic layer, whichsatisfies the equation:E ₂=(M _(s2) H _(k2)Ω₂)/2, wherein:

M_(s2)=saturation magnetization of the free magnetic layer;

H_(k2)=anisotropy field of the free magnetic layer; and

Ω₂=volume of the free magnetic layer.

The time T_(IEC) for switching the magnetization direction of the freemagnetic layer 704 in response to the input voltage V_(in) between thefixed magnetic layer 702 and the free magnetic layer 704 satisfies theswitching speed equation in FIG. 10:T _(IEC)=((8(√π)|J _(IEC0)|)/(α_(g)γ(H _(k2) +n2πM _(s2))|J_(IEC)|))(π/2−θ₀), wherein:

J_(IEC0)=energy density threshold of IEC needed for switching themagnetization direction in the free magnetic layer;

α_(g)=the Gilbert damping factor of the free magnetic layer;

γ=the electron gyromagnetic ratio in units of radian per second pertesla;

H_(k2)=the anisotropy field of the free magnetic layer;

J_(IEC)=the induced IEC energy density due to electric field created byapplied input voltage;

θ₀=the initial angle between the directions of magnetization of thefixed magnetic layer and the free magnetic layer in the units ofradians;

n=0 for a free magnet with perpendicular anisotropy; and

n=1 for a free magnet with in-plane anisotropy.

The equation above for determining the minimum switching time T_(IEC),measured in units of seconds, includes the Gilbert damping factor α_(g)and the parameter n that determines in-plane (when n=1) or perpendicular(when n=0) anisotropy of the free magnetic layer 704, but the equationfor determining the IEC energy threshold J_(IEC0)) does not. Thiscontrasts to the situation of STT MRAM, in which the current densityrequired to switch a magnetization direction depends on both the Gilbertdamping factor α_(g) and the parameter n that determines whether thefree magnetic layer has in-plane or perpendicular anisotropy. The energydensity required for switching in STT is directly proportional to theGilbert damping factor, but the minimum switching time is inverselyproportional to the Gilbert damping factor α_(g). Thus, reducing theenergy density (i.e., current density) required to switch amagnetization direction in an STT type MRAM increases the minimum timefor switching. Thus, STT suffers from a trade-off between currentdensity and switching time. In the magnetic memory structure 700,however, reducing the energy density (i.e., threshold IEC, J_(IEC0))needed to switch the magnetization direction of the free layer 704 canalso reduce the minimum switching time T_(IEC).

The STT switching threshold is proportional to E₂, whereas the IECswitching threshold J_(IEC0) is proportional to an effective combinationof E₁ and E₂. Thus, the effective combination yields a value <E₂ andreaches E₂ as a maximum value when E₁>>E₂. As apparent in the aboveequations, both IEC and STT switching speeds depend on the Gilbertdamping factor α_(g) and the parameter n of the free magnetic layer thatdetermines whether the free magnetic layer has an in-plane or aperpendicular anisotropy, and both switching speeds can be reduced withan increase in the applied energy density (e.g., when J_(IEC)>J_(IEC0)).

Unlike STT, in which switching speed depends on the initial angle θ₀between the magnetization directions of the fixed magnetic layer and thefree magnetic layer in a logarithmic way, IEC switching speed islinearly dependent on the initial angle θ₀.

Some of the variables in the equations above are determined by thematerials of which the respective layers of the magnetic memorystructure 700 are formed and the dimensions of those respective layers.In the example in the simulation illustrated in FIG. 8B, the fixedmagnetic layer 702 is formed of -cobalt (Co) having a thickness of 10nm. The free magnetic layer 704 is formed of Co having a thickness of 1nm. The oxide layers 710 are formed of magnesium oxide (MgO) having athickness of about 1 nm. The spacer layer 708 is formed of ruthenium(Ru) having a thickness of 0.8 nm.

However, the magnetic memory structure 700 disclosed herein is notlimited to the materials and dimensions of the example of FIG. 8B. Forexample, the fixed magnetic layer 702 and the free magnetic layer 704may each be formed from any one of iron (Fe), cobalt (Co), nickel (Ni),and an alloy of Fe, Co or Ni. The fixed magnetic layer 702 may have athickness in the range of 5 to 100 nm, and the free magnetic layer 704may have a thickness in the range of 1 to 3 nm.

The first and second oxide layers 710 may each be formed of any one ofMgO, magnesium zinc oxide (MgZnO), aluminum monoxide (AlOx) (thermallygrown), iron oxide (Fe2O3), titanium oxide (TiO2), zinc oxide (ZnO),niobium oxide (Nb2O5), rhodium oxide (Rh2O3), tantalum oxide (Ta2O3),vanadium oxide (V2O5), aluminum oxide (Al2O3), and hafnium oxide (HfO2).The first and second oxide layers 710 may each have a thickness in therange of 0.5 to 3 nm.

The spacer layer 708 may be formed of any one of ruthenium (Ru), gold(Au), iridium (Ir), rhodium (Rh), copper (Cu), chromium (Cr), palladium(Pd), molybdenum (Mo), vanadium (V), tantalum (Ta), tungsten (W),platinum (Pt), nickel oxide (NiO), and iron silicon (FeSi). The spacerlayer 708 may have a thickness in the range of 0.1 to 4 nm.

In another example, a method of setting the magnetization direction ofthe free magnetic layer 704 in the magnetic memory structure 700includes supplying a first input voltage V_(in) of a polarity betweenthe fixed magnetic layer 702 and the free magnetic layer 704, which areseparated by a composite layer, to control the magnetization directionof the free magnetic layer 704 to be parallel to the magnetizationdirection of the fixed magnetic layer 702, and supplying a second inputvoltage V_(in) of the polarity between the fixed magnetic layer 702 andthe free magnetic layer 704 to control the magnetization direction ofthe free magnetic layer 704 to be antiparallel to the magnetizationdirection of the fixed magnetic layer 702. The time T_(IEC) required forsupplying the input voltage V_(in) to set the magnetization direction ofthe free magnetic layer 704 relative to the magnetization direction ofthe fixed magnetic layer 702 is directly related to the magnitude of theinput voltage V_(in) supplied.

FIG. 11A is a graphical representation showing that the peaks of theoscillatory IEC appear at lower input voltage V_(in) with an increasingwidth of the QW 716, which corresponds to the thickness of the spacerlayer 708. FIG. 11A also shows that the magnitude of the input voltageV_(in) required to achieve a peak (positive or negative) in themagnitude of the IEC is inversely related to the thickness of the spacerlayer 708. In other words, the magnitude of the input voltage decreasesas the thickness of the spacer layer 708 is increased. Thus, in themagnetic memory structure 700, the magnetization direction of the freemagnetic layer 704 relative to the magnetization direction of the fixedmagnetic layer 702 corresponds to the magnitude of the input voltageV_(in) between the fixed magnetic layer 702 and the free magnetic layer704, and the magnitude of the input voltage V_(in) is inversely relatedto the thickness of the spacer layer 708. However, increasing thethickness of the spacer layer 708 increases the distance between thefixed magnetic layer 702 and the free magnetic layer 704; hence the IECstrength decreases.

Thus, in the magnetic memory structure 700, a first magnetizationdirection of the free magnetic layer 704 relative to the magnetizationdirection of the fixed magnetic layer 702 (e.g., parallel orantiparallel) corresponds to a first magnitude of an input voltageV_(in) of a polarity (e.g., positive or negative) between the fixedmagnetic layer 702 and the free magnetic layer 704. For example, at a QWwidth of 0.8 nm, a negative peak of IEC is observed at an input voltageV_(in) magnitude of approximately 1.2 V. A second magnetizationdirection of the free magnetic layer 704 relative to the magnetizationdirection of the fixed magnetic layer 702 (e.g., antiparallel orparallel) corresponds to a second magnitude of the input voltage V_(in)of the same polarity (e.g., positive or negative) between the fixedmagnetic layer 702 and the free magnetic layer 704. For example, at theQW width of 0.8 nm, a positive peak of IEC is observed at an inputvoltage V_(in) magnitude of approximately 1.5 V. As shown in FIG. 11A,at a QW width of 0.9 nm, a negative peak of IEC is observed at an inputvoltage V_(in) magnitude of approximately 1.1 V, and a positive peak ofIEC is observed at an input voltage V_(in) magnitude of approximately1.3 V. Thus, the first magnitude of the input voltage V_(in) and thesecond magnitude of the input voltage V_(in) depend on the QW width,which corresponds to a thickness of the spacer layer 708. Specifically,since the input voltage magnitudes for observing peaks decrease with anincrease in QW width, the first magnitude of the input voltage V_(in)and the second magnitude of the input voltage V_(in) are inverselyrelated to the thickness of the spacer layer 708.

FIG. 11B is a graphical representation showing that the input voltageV_(in) required to maintain a particular magnitude of IEC (i.e., J_(ex))increases with the height of the QW 716. That is, the magnitude of theIEC diminishes with an increase in height of the QW 716, even in thepresence of a higher input voltage V. Thus, FIGS. 11A and 11B illustratehow the height and width of the QW 716 can be used to adjust a range ofthe input voltage V_(in), the magnitude of the IEC, and the currentdensity in the structure.

The electrical resistance of the magnetic memory structure 700 in thecondition that the magnetization directions of the fixed and freemagnetic layers 702 and 704 are antiparallel is higher than theelectrical resistance of the magnetic memory structure 700 in thecondition that the magnetization directions of the fixed and freemagnetic layers 702 and 704 are parallel. The parallel or antiparallelstate represents a binary value, and binary data is written into themagnetic memory structure 700 by setting the relative magnetizationdirections to be parallel or antiparallel, as discussed above. The data(e.g., binary value) stored in the magnetic memory structure 700 may bedetermined by several means, as described with regard to FIGS. 12-14.

In the configuration of the magnetic memory structure 700 illustrated inFIG. 12, the fixed magnetic layer 702 and the free magnetic layer 704have perpendicular magnetic anisotropy (pma). The free magnetic layer704 is deposited on a Hall bar geometry. A current is run in one of thehands of the hall bar and an anomalous Hall voltage is measured acrossthe other hand perpendicular to the current flow, which is perpendicularto the current flowing hand. In order to read the magnetization usingthe anomalous Hall voltage, the easy axis of the free magnetizationshould be perpendicular to the both hands of the hall bar, hence, a freemagnet with perpendicular anisotropy is required for this read-outmethod. Depending on whether the free magnetic layer 704 is pointingoutward from the Hall bar plane or inward toward the Hall bar plane, themeasured Hall voltage will be high or low, respectively. Note that thereading of the free magnetic layer information does not depend on thefixed magnetic layer configuration of the magnetic memory structure 700shown in FIG. 12.

In the configuration of the magnetic memory structure 700 illustrated inFIG. 13, a fixed read current input “I” is passed across the end of thefree magnetic layer 704. Depending on a direction of magnetization ofthe free magnetic layer 704, the magnetic field therein creates avoltage differential V_(DIFF) between V₁ and V₂. This apparatus uses amethod known as the Hall effect. By measuring the voltage V_(DIFF), thedirection of magnetization (i.e., the state of the magnetic memorystructure 700) can be determined.

In the configuration illustrated in FIG. 14, in a case in which themagnetoresistance of the magnetic memory structure 700 may be too low toeasily distinguish between the antiparallel and parallel states ofmagnetization directions, additional layers such as oxide layer 1402 andreference layer 1404 can be added to enhance the difference inmagnetoresistance between the respective states. A voltage appliedbetween the contacts 1406 and 1408 can be used to determine themagnetoresistance, and thus the state of the magnetic memory structure700.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A magnetic memory structure, comprising: a fixedmagnetic layer; a free magnetic layer; and a non-magnetic compositelayer disposed between the fixed magnetic layer and the free magneticlayer; wherein a magnetization direction of the free magnetic layerrelative to a magnetization direction of the fixed magnetic layer is:parallel in response to a first input voltage of a polarity between thefixed magnetic layer and the free magnetic layer, and antiparallel inresponse to a second input voltage of the polarity between the fixedmagnetic layer and the free magnetic layer.
 2. The magnetic memorystructure of claim 1, wherein: a magnitude of the first input voltage isdifferent than a magnitude of the second input voltage.
 3. The magneticmemory structure of claim 1, wherein the magnetic memory structure isconfigured to switch the magnetization direction of the free magneticlayer relative to the magnetization direction of the fixed magneticlayer from antiparallel to parallel in response to the first inputvoltage, and from parallel to antiparallel in response to the secondinput voltage.
 4. The magnetic memory structure of claim 1, wherein thenon-magnetic composite layer comprises: a non-magnetic spacer layerdisposed between the fixed magnetic layer and the free magnetic layer; afirst oxide layer disposed between the fixed magnetic layer and thespacer layer; and a second oxide layer disposed between the spacer layerand the free magnetic layer.
 5. The magnetic memory structure of claim1, wherein: each of the fixed magnetic layer and the free magnetic layercomprises any one of iron (Fe), cobalt (Co), nickel (Ni), and an alloyof Fe, Co or Ni.
 6. The magnetic memory structure of claim 4, wherein:the first oxide layer and the second oxide layer each comprise any oneof magnesium oxide (MgO), magnesium zinc oxide (MgZnO), aluminummonoxide (AlOx) (thermally grown), iron oxide (Fe2O3), titanium oxide(TiO2), zinc oxide (ZnO), niobium oxide (Nb2O5), rhodium oxide (Rh2O3),tantalum oxide (Ta2O3), vanadium oxide (V2O5), aluminum oxide (Al2O3),and hafnium oxide (HfO2).
 7. The magnetic memory structure of claim 4,wherein: the spacer layer comprises any one of ruthenium (Ru), gold(Au), iridium (Ir), rhodium (Rh), copper (Cu), chromium (Cr), palladium(Pd), molybdenum (Mo), vanadium (V), tantalum (Ta), tungsten (W),platinum (Pt), nickel oxide (NiO), and iron silicon (FeSi).
 8. Themagnetic memory structure of claim 1, further comprising: a firstelectrode electrically coupled to the fixed magnetic layer; and a secondelectrode electrically coupled to the free magnetic layer; wherein thefirst input voltage and the second input voltage are each appliedbetween the first electrode and the second electrode.
 9. The magneticmemory structure of claim 1, wherein the first input voltage and thesecond input voltage increase interlayer exchange coupling (IEC) betweenthe fixed magnetic layer and the free magnetic layer above a thresholdat which the magnetization direction of the free magnetic layer isdetermined by the magnetization direction of the fixed magnetic layer.10. The magnetic memory structure of claim 4, wherein: the fixedmagnetic layer has a thickness of 10 nanometers (nm); the free magneticlayer has a thickness of 1 nm; the first oxide layer and the secondoxide layer each have a thickness of 1 nm; and the spacer layer has athickness of 0.8 nm.
 11. A magnetic memory structure, comprising: afixed magnetic layer; a free magnetic layer; and a composite layerproviding a quantum well between the fixed magnetic layer and the freemagnetic layer; wherein: a magnetization direction of the free magneticlayer relative to a magnetization direction of the fixed magnetic layercorresponds to an interlayer exchange coupling (IEC) in the compositelayer based on a magnitude of an input voltage between the fixedmagnetic layer and the free magnetic layer.
 12. The magnetic memorystructure of claim 11, wherein: an IEC energy density threshold forswitching the magnetization direction of the free magnetic layerrelative to the magnetization direction of the fixed magnetic layer isindependent of a Gilbert damping factor of the free magnetic layer. 13.The magnetic memory structure of claim 11, wherein: an IEC energydensity threshold for switching the magnetization direction of the freemagnetic layer relative to the magnetization direction of the fixedmagnetic layer is independent of a parameter that indicates the magneticanisotropy of the free magnetic layer being in-plane or perpendicular.14. The magnetic memory structure of claim 11, wherein the IEC based onthe magnitude of the input voltage between the fixed magnetic layer andthe free magnetic layer comprises an energy density J_(IEC0) thatsatisfies the equation:J _(IEC0)=(E ₁ ×E ₂)/(S(E ₁ +E ₂)), wherein: J_(IEC0)=minimum energydensity threshold of the IEC needed for switching the magnetizationdirection of the free magnetic layer, and is measured in units of Jouleper square meter; S=cross-sectional area of the magnetic memorystructure; E₁=thermal energy barrier of the fixed magnetic layer, whichsatisfies the equation:E ₁=(M _(s1) H _(k1)Ω₁)/2, wherein: M_(s1)=saturation magnetization ofthe fixed magnetic layer; H_(k1)=anisotropy field of the fixed magneticlayer; and Ω₁=volume of the fixed magnetic layer; and E₂=thermal energybarrier of the free magnetic layer, which satisfies the equation:E ₂=(M _(s2) H _(k2)Ω₂)/2, wherein: M_(s2)=saturation magnetization ofthe free magnetic layer; H_(k2)=anisotropy field of the free magneticlayer; and Ω₂=volume of the free magnetic layer.
 15. The magnetic memorystructure of claim 11, wherein a time for switching the magnetizationdirection of the free magnetic layer in response to the input voltagebetween the fixed magnetic layer and the free magnetic layer satisfiesthe equation:T _(IEC)=((8(π⁻¹)|J _(IEC0)|)/(α_(g)γ(H _(k2) +n2πM _(s2))|J_(IEC0)|(π/2−θ₀), wherein: J_(IEC0)=energy density threshold ofinterlayer exchange coupling (IEC) needed for switching themagnetization direction in the free magnetic layer; α_(g)=Gilbertdamping factor of the free magnetic layer; γ=electron gyromagnetic ratioin units of radian per second per tesla; H_(k2)=anisotropy field of thefree magnetic layer; J_(IEC0)=induced IEC energy density due to electricfield created by applied input voltage; θ₀=initial angle between themagnetization directions of the fixed magnetic layer and the freemagnetic layer in units of radians; n=0 for a free magnet withperpendicular anisotropy; and n=1 for a free magnet with in-planeanisotropy.
 16. The magnetic memory structure of claim 15, wherein: themagnetization direction of the free magnetic layer relative to themagnetization direction of the fixed magnetic layer is: parallel inresponse to a first input voltage between the fixed magnetic layer andthe free magnetic layer, and antiparallel in response to a second inputvoltage between the fixed magnetic layer and the free magnetic layer.17. The magnetic memory structure of claim 11, wherein: each of thefixed magnetic layer and the free magnetic layer comprises any one ofiron (Fe), cobalt (Co), nickel (Ni), and an alloy of Fe, Co or Ni. 18.The magnetic memory structure of claim 11, wherein the composite layercomprises: a spacer layer providing the quantum well and disposedbetween the fixed magnetic layer and the free magnetic layer; a firstoxide layer disposed between the fixed magnetic layer and the spacerlayer; and a second oxide layer disposed between the spacer layer andthe free magnetic layer.
 19. The magnetic memory structure of claim 18,wherein: the first oxide layer and the second oxide layer each compriseany one of magnesium oxide (MgO), magnesium zinc oxide (MgZnO), aluminummonoxide (AlOx) (thermally grown), iron oxide (Fe2O3), titanium oxide(TiO2), zinc oxide (ZnO), niobium oxide (Nb2O5), rhodium oxide (Rh2O3),tantalum oxide (Ta2O3), vanadium oxide (V2O5), aluminum oxide (Al2O3),and hafnium oxide (HfO2).
 20. The magnetic memory structure of claim 18,wherein: the spacer layer comprises any one of ruthenium (Ru), gold(Au), iridium (Ir), rhodium (Rh), copper (Cu), chromium (Cr), palladium(Pd), molybdenum (Mo), vanadium (V), tantalum (Ta), tungsten (W),platinum (Pt), nickel oxide (NiO), and iron silicon (FeSi).
 21. Amagnetic memory structure, comprising: a fixed magnetic layer; a freemagnetic layer; and a composite layer disposed between the fixedmagnetic layer and the free magnetic layer, the composite layercomprising: a spacer layer disposed between the fixed magnetic layer andthe free magnetic layer; a first oxide layer disposed between the fixedmagnetic layer and the spacer layer; and a second oxide layer disposedbetween the spacer layer and the free magnetic layer; wherein: thespacer layer provides a large interlayer exchange coupling (IEC) betweenthe fixed magnetic layer and the free magnetic layer relative to thefirst oxide layer and the second oxide layer; a first magnetizationdirection of the free magnetic layer relative to a magnetizationdirection of the fixed magnetic layer corresponds to a first magnitudeof an input voltage of a polarity between the fixed magnetic layer andthe free magnetic layer; a second magnetization direction of the freemagnetic layer relative to the magnetization direction of the fixedmagnetic layer corresponds to a second magnitude of the input voltage ofthe polarity between the fixed magnetic layer and the free magneticlayer; and the first magnitude of the input voltage and the secondmagnitude of the input voltage depend on a thickness of the spacerlayer.
 22. The magnetic memory structure of claim 21, wherein: the firstmagnitude of the input voltage and the second magnitude of the inputvoltage are inversely related to the thickness of the spacer layer. 23.The magnetic memory structure of claim 21, wherein: each of the fixedmagnetic layer and the free magnetic layer comprises any one of iron(Fe), cobalt (Co), nickel (Ni), and an alloy of Fe, Co or Ni.
 24. Themagnetic memory structure of claim 21, wherein: the first oxide layerand the second oxide layer each comprise any one of magnesium oxide(MgO), magnesium zinc oxide (MgZnO), aluminum monoxide (AlOx) (thermallygrown), iron oxide (Fe2O3), titanium oxide (TiO2), zinc oxide (ZnO),niobium oxide (Nb2O5), rhodium oxide (Rh2O3), tantalum oxide (Ta2O3),vanadium oxide (V2O5), aluminum oxide (Al2O3), and hafnium oxide (HfO2).25. The magnetic memory structure of claim 21, wherein: the spacer layercomprises any one of ruthenium (Ru), gold (Au), iridium (Ir), rhodium(Rh), copper (Cu), chromium (Cr), palladium (Pd), molybdenum (Mo),vanadium (V), tantalum (Ta), tungsten (W), platinum (Pt), nickel oxide(NiO), and iron silicon (FeSi).
 26. A method of storing data in amagnetic memory structure, comprising: supplying a first input voltageof a polarity between a fixed magnetic layer and a free magnetic layerseparated by a composite layer providing a quantum well to control amagnetization direction of the free magnetic layer to be parallel to amagnetization direction of the fixed magnetic layer; and supplying asecond input voltage of the polarity between the fixed magnetic layerand the free magnetic layer to control the magnetization direction ofthe free magnetic layer to be antiparallel to the magnetizationdirection of the fixed magnetic layer.